ABSTRACT
The first structure depicted in Figure 34.3b is that of a plasma polymer film deposited on the porous substrate coated with conventional polymer. The usefulness of this technique is that a conventional coating that has good casting or coating properties can be used to cover the surface pores of the substrate. A nanofilm of a plasma polymer can then be deposited on the coating to improve the selectivity. This is particularly useful when the surface pores of the substrate are too large to be covered by a plasma polymer film thin enough to take advantage of the nanofilm technology of LCVD. The gas permeability obtained with this type of composite membrane is summarized in Table 34.3 [2]. Although the composite membranes prepared did not show a permeability ratio of H2/CH4 as high as those using homogeneous (nonporous) silicon-carbonate sheet as the substrate (Table 34.2), the hydrogen permeability of the composite membranes is one to two orders of magnitude higher than that obtained with thin silicone-carbonate sheets. A substantial increase in the permeation rate obtained with these double-coated membranes, while retaining a reasonable separation factor, is a unique advantage of the application of nanoscale molecular sieve obtainable by LCVD.
